Safari lags on brand new computer.

The warranty entitles you to complimentary phone support for the first 90 days of ownership.

If you bought the product in the U.S. directly from Apple (not from a reseller), you have 14 days from the date of delivery in which to exchange or return it for a refund. In other countries, the return policy may be different. If you bought from a reseller, its return policy applies.

Third-party Safari extensions can slow Safari.

So can anti-virus and so-called "cleaning/optimizing/tune-up" apps. The are not worth it, even if free.

Do you have anything like those running?

are you sure its your computer at all do a speed test on your internet http://www.speedtest.net/ if you still have an issue reset both your modem and router. not all problems are the computer itself.

Safari - Unsupported third-party add-ons may cause Safari to unexpectedly quit or have performance issues

Safari doesn't load a page or webpage items are missing

Safari/other browsers – Website not loading

I have same problem, new macbook air CTO 2015 and waiting the tab to open in safari like 30seconds, sometimes when i wanna type link i also have to wait around 30 seconds, pretty overpriced products from apple ...they still didnt solved my problem, also bottom of my macbook is coming out